In internal combustion engines, such as 4-stroke motorcycle engines, a small portion of exhaust gases that are present after ignition of the air/fuel mixture in a cylinder of the engine penetrates between the cylinder and the piston into the crankcase. The amount of such “blow-by” usually increases as the engine wears.
The blow-by gas results in an increase of pressure in the crankcase that needs to be reduced in order to reduce loss of power or torque of the engine and consequently the crankcase typically is ventilated. In order to reduce toxic emission of the engine it is usually not permissible to ventilate the crankcase directly to the environment. Crankcase ventilation therefore usually comprises a tube that is connected with one end to a portion of the crankcase and with the other end to an air-filter element of the engine.
However, usually an oil-vapour is also emitted from the crankcase together with the exhaust and the ejected oil vapour will contaminate and eventually block-up the air-filter element. If the air filter element is contaminated, oil will typically be emitted directly into the environment.